A call has been made for the contractor who dumped three bags of 1080 at Rakiura National Park in Stewart Island to be prosecuted.
Invercargill MP Sarah Dowie said she was expecting a prosecution because the action was a serious breach of trust.
"I've contacted the director general of conservation. I've also drafted up letters to WorkSafe, who administer compliance with regards to hazardous substances," she said.
"I've written to the Department of Conservation and also to the [Southland Regional Council]. I'm expecting full investigations, I'm expecting a prosecution, and I'm expecting a review of procedures as to exactly how this happened."
The Department of Conservation (DOC) said a contractor left 75kg of 1080 in a remote backcountry block at Rakiura National Park during a four-month ground baiting operation, before it was removed yesterday.
The operation started in March and finished at the end of June.
Ms Dowie said she would like to know what prompted the contractor to dump the poison.
"What was this experienced guy thinking - did he think that he could get away with it?" she said.
"I'm not sure what's gone through this contractor's head. 1080 is a poisonous substance, it's the best that we've got with regards to conservation efforts at the moment.
"There is a high level of trust in using that chemical. The fact that it's been illegally dumped is just absolutely unacceptable, and we need to find out exactly why he felt empowered to do it."
DOC's acting deputy director general for operations Reg Kemper said the bags of possum cereal bait contained a small amount of 1080, which breaks down quickly, doesn't pose any risks to humans and no native animals would have been tempted to eat it.
He said it beggared belief that a qualified professional hired to do pest control would think it was acceptable to dump toxin.
"I am disappointed and angry that the important pest control work carried out by DOC has been undermined by the actions of an individual contractor."
DOC launched an investigation and reported the dumping to the Environment Protection Authority, the police, the Southland Regional Council and the Medical Officer of Health.
Police said they were aware of the matter, but they have not launched an investigation yet.